Struct memchr::memmem::searcher::PrefilterState

pub(crate) struct PrefilterState {
    skips: u32,
    skipped: u32,
}

PrefilterState tracks state associated with the effectiveness of a prefilter. It is used to track how many bytes, on average, are skipped by the prefilter. If this average dips below a certain threshold over time, then the state renders the prefilter inert and stops using it.

A prefilter state should be created for each search. (Where creating an iterator is treated as a single search.) A prefilter state should only be created from a Freqy. e.g., An inert Freqy will produce an inert PrefilterState.

Fields

skips: u32

The number of skips that has been executed. This is always 1 greater than the actual number of skips. The special sentinel value of 0 indicates that the prefilter is inert. This is useful to avoid additional checks to determine whether the prefilter is still “effective.” Once a prefilter becomes inert, it should no longer be used (according to our heuristics).

skipped: u32

The total number of bytes that have been skipped.

Implementations

impl PrefilterState

const MIN_SKIPS: u32 = 50u32

The minimum number of skip attempts to try before considering whether a prefilter is effective or not.

const MIN_SKIP_BYTES: u32 = 8u32

The minimum amount of bytes that skipping must average.

This value was chosen based on varying it and checking the microbenchmarks. In particular, this can impact the pathological/repeated-{huge,small} benchmarks quite a bit if it’s set too low.

pub(crate) fn new() -> PrefilterState

Create a fresh prefilter state.

fn update(&mut self, skipped: usize)

Update this state with the number of bytes skipped on the last invocation of the prefilter.

fn is_effective(&mut self) -> bool

Return true if and only if this state indicates that a prefilter is still effective.

fn is_inert(&self) -> bool

Returns true if the prefilter this state represents should no longer be used.

fn skips(&self) -> u32

Returns the total number of times the prefilter has been used.

Trait Implementations

impl Clone for PrefilterState

fn clone(&self) -> PrefilterState

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for PrefilterState

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Copy for PrefilterState